[SimplifyCFG] Use range check in simplifyBranchOnICmpChain if possible #165105
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This was referenced Oct 25, 2025
| @llvm/pr-subscribers-llvm-transforms Author: Yingwei Zheng (dtcxzyw) ChangesIn Note: I cannot guarantee that such an infinite loop cannot be encountered after this patch. FWICT, we don't have a method to efficiently detect this bug for CFG optimizations (For peephole optimizations, see Termination-Checking for LLVM Peephole Optimizations). Closes #165088. Full diff: https://github.com/llvm/llvm-project/pull/165105.diff 2 Files Affected:
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp index d831c2737e5f8..50272ddc46699 100644 --- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp +++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp @@ -5228,32 +5228,53 @@ bool SimplifyCFGOpt::simplifyBranchOnICmpChain(BranchInst *BI, CompVal, DL.getIntPtrType(CompVal->getType()), "magicptr"); } - // Create the new switch instruction now. - SwitchInst *New = Builder.CreateSwitch(CompVal, DefaultBB, Values.size()); - if (HasProfile) { - // We know the weight of the default case. We don't know the weight of the - // other cases, but rather than completely lose profiling info, we split - // the remaining probability equally over them. - SmallVector<uint32_t> NewWeights(Values.size() + 1); - NewWeights[0] = BranchWeights[1]; // this is the default, and we swapped if - // TrueWhenEqual. - for (auto &V : drop_begin(NewWeights)) - V = BranchWeights[0] / Values.size(); - setBranchWeights(*New, NewWeights, /*IsExpected=*/false); - } - - // Add all of the 'cases' to the switch instruction. - for (ConstantInt *Val : Values) - New->addCase(Val, EdgeBB); - - // We added edges from PI to the EdgeBB. As such, if there were any - // PHI nodes in EdgeBB, they need entries to be added corresponding to - // the number of edges added. - for (BasicBlock::iterator BBI = EdgeBB->begin(); isa<PHINode>(BBI); ++BBI) { - PHINode *PN = cast<PHINode>(BBI); - Value *InVal = PN->getIncomingValueForBlock(BB); - for (unsigned i = 0, e = Values.size() - 1; i != e; ++i) - PN->addIncoming(InVal, BB); + // Check if we can represent the values can be represented as a contiguous + // range. If so, we use a range check + conditional branch instead of a + // switch. + if (Values.front()->getValue() - Values.back()->getValue() == + Values.size() - 1) { + ConstantRange RangeToCheck = ConstantRange::getNonEmpty( + Values.back()->getValue(), Values.front()->getValue() + 1); + APInt Offset, RHS; + ICmpInst::Predicate Pred; + RangeToCheck.getEquivalentICmp(Pred, RHS, Offset); + Value *X = CompVal; + if (!Offset.isZero()) + X = Builder.CreateAdd(X, ConstantInt::get(CompVal->getType(), Offset)); + Value *Cond = + Builder.CreateICmp(Pred, X, ConstantInt::get(CompVal->getType(), RHS)); + BranchInst *NewBI = Builder.CreateCondBr(Cond, EdgeBB, DefaultBB); + if (HasProfile) + setBranchWeights(*NewBI, BranchWeights, /*IsExpected=*/false); + // We don't need to update PHI nodes since we don't add any new edges. + } else { + // Create the new switch instruction now. + SwitchInst *New = Builder.CreateSwitch(CompVal, DefaultBB, Values.size()); + if (HasProfile) { + // We know the weight of the default case. We don't know the weight of the + // other cases, but rather than completely lose profiling info, we split + // the remaining probability equally over them. + SmallVector<uint32_t> NewWeights(Values.size() + 1); + NewWeights[0] = BranchWeights[1]; // this is the default, and we swapped + // if TrueWhenEqual. + for (auto &V : drop_begin(NewWeights)) + V = BranchWeights[0] / Values.size(); + setBranchWeights(*New, NewWeights, /*IsExpected=*/false); + } + + // Add all of the 'cases' to the switch instruction. + for (ConstantInt *Val : Values) + New->addCase(Val, EdgeBB); + + // We added edges from PI to the EdgeBB. As such, if there were any + // PHI nodes in EdgeBB, they need entries to be added corresponding to + // the number of edges added. + for (BasicBlock::iterator BBI = EdgeBB->begin(); isa<PHINode>(BBI); ++BBI) { + PHINode *PN = cast<PHINode>(BBI); + Value *InVal = PN->getIncomingValueForBlock(BB); + for (unsigned i = 0, e = Values.size() - 1; i != e; ++i) + PN->addIncoming(InVal, BB); + } } // Erase the old branch instruction. diff --git a/llvm/test/Transforms/SimplifyCFG/pr165088.ll b/llvm/test/Transforms/SimplifyCFG/pr165088.ll new file mode 100644 index 0000000000000..4514a1927b586 --- /dev/null +++ b/llvm/test/Transforms/SimplifyCFG/pr165088.ll @@ -0,0 +1,186 @@ +; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6 +; RUN: opt -S -passes="simplifycfg<switch-range-to-icmp>" < %s | FileCheck %s + +; Avoid getting stuck in the cycle pr165088_cycle_[1-4]. + +define void @pr165088_cycle_1(i8 %x) { +; CHECK-LABEL: define void @pr165088_cycle_1( +; CHECK-SAME: i8 [[X:%.*]]) { +; CHECK-NEXT: [[ENTRY:.*:]] +; CHECK-NEXT: [[TMP0:%.*]] = icmp ult i8 [[X]], 2 +; CHECK-NEXT: br i1 [[TMP0]], label %[[BLOCK2:.*]], label %[[BLOCK3:.*]] +; CHECK: [[BLOCK1:.*]]: +; CHECK-NEXT: [[COND2:%.*]] = icmp ugt i8 [[X]], 1 +; CHECK-NEXT: br i1 [[COND2]], label %[[BLOCK3]], label %[[BLOCK2]] +; CHECK: [[BLOCK2]]: +; CHECK-NEXT: br label %[[BLOCK3]] +; CHECK: [[BLOCK3]]: +; CHECK-NEXT: [[COND3:%.*]] = icmp eq i8 [[X]], 0 +; CHECK-NEXT: br i1 [[COND3]], label %[[EXIT:.*]], label %[[BLOCK1]] +; CHECK: [[EXIT]]: +; CHECK-NEXT: ret void +; +entry: + %switch = icmp uge i8 %x, 2 + %cond1 = icmp ugt i8 %x, 1 + %or.cond = and i1 %switch, %cond1 + br i1 %or.cond, label %block3, label %block2 + +block1: + %cond2 = icmp ugt i8 %x, 1 + br i1 %cond2, label %block3, label %block2 + +block2: + br label %block3 + +block3: + %cond3 = icmp eq i8 %x, 0 + br i1 %cond3, label %exit, label %block1 + +exit: + ret void +} + +define void @pr165088_cycle_2(i8 %x) { +; CHECK-LABEL: define void @pr165088_cycle_2( +; CHECK-SAME: i8 [[X:%.*]]) { +; CHECK-NEXT: [[ENTRY:.*:]] +; CHECK-NEXT: [[SWITCH:%.*]] = icmp ult i8 [[X]], 2 +; CHECK-NEXT: br i1 [[SWITCH]], label %[[BLOCK2:.*]], label %[[BLOCK3:.*]] +; CHECK: [[BLOCK1:.*]]: +; CHECK-NEXT: [[COND2:%.*]] = icmp ugt i8 [[X]], 1 +; CHECK-NEXT: br i1 [[COND2]], label %[[BLOCK3]], label %[[BLOCK2]] +; CHECK: [[BLOCK2]]: +; CHECK-NEXT: br label %[[BLOCK3]] +; CHECK: [[BLOCK3]]: +; CHECK-NEXT: [[COND3:%.*]] = icmp eq i8 [[X]], 0 +; CHECK-NEXT: br i1 [[COND3]], label %[[EXIT:.*]], label %[[BLOCK1]] +; CHECK: [[EXIT]]: +; CHECK-NEXT: ret void +; +entry: + switch i8 %x, label %block3 [ + i8 1, label %block2 + i8 0, label %block2 + ] + +block1: ; preds = %block3 + %cond2 = icmp ugt i8 %x, 1 + br i1 %cond2, label %block3, label %block2 + +block2: ; preds = %entry, %entry, %block1 + br label %block3 + +block3: ; preds = %entry, %block2, %block1 + %cond3 = icmp eq i8 %x, 0 + br i1 %cond3, label %exit, label %block1 + +exit: ; preds = %block3 + ret void +} + +define void @pr165088_cycle_3(i8 %x) { +; CHECK-LABEL: define void @pr165088_cycle_3( +; CHECK-SAME: i8 [[X:%.*]]) { +; CHECK-NEXT: [[ENTRY:.*:]] +; CHECK-NEXT: br label %[[BLOCK3:.*]] +; CHECK: [[BLOCK3]]: +; CHECK-NEXT: [[COND3:%.*]] = icmp eq i8 [[X]], 0 +; CHECK-NEXT: br i1 [[COND3]], label %[[EXIT:.*]], label %[[BLOCK3]] +; CHECK: [[EXIT]]: +; CHECK-NEXT: ret void +; +entry: + switch i8 %x, label %block1 [ + i8 1, label %block2 + i8 0, label %block2 + ] + +block1: ; preds = %entry, %block3 + %cond2 = icmp ugt i8 %x, 1 + br i1 %cond2, label %block3, label %block2 + +block2: ; preds = %entry, %entry, %block1 + br label %block3 + +block3: ; preds = %block2, %block1 + %cond3 = icmp eq i8 %x, 0 + br i1 %cond3, label %exit, label %block1 + +exit: ; preds = %block3 + ret void +} + +define void @pr165088_cycle_4(i8 %x) { +; CHECK-LABEL: define void @pr165088_cycle_4( +; CHECK-SAME: i8 [[X:%.*]]) { +; CHECK-NEXT: [[ENTRY:.*:]] +; CHECK-NEXT: [[TMP0:%.*]] = icmp ult i8 [[X]], 2 +; CHECK-NEXT: br i1 [[TMP0]], label %[[BLOCK2:.*]], label %[[BLOCK3:.*]] +; CHECK: [[BLOCK1:.*]]: +; CHECK-NEXT: [[COND2_OLD:%.*]] = icmp ugt i8 [[X]], 1 +; CHECK-NEXT: br i1 [[COND2_OLD]], label %[[BLOCK3]], label %[[BLOCK2]] +; CHECK: [[BLOCK2]]: +; CHECK-NEXT: br label %[[BLOCK3]] +; CHECK: [[BLOCK3]]: +; CHECK-NEXT: [[COND3:%.*]] = icmp eq i8 [[X]], 0 +; CHECK-NEXT: br i1 [[COND3]], label %[[EXIT:.*]], label %[[BLOCK1]] +; CHECK: [[EXIT]]: +; CHECK-NEXT: ret void +; +entry: + %switch = icmp ult i8 %x, 2 + br i1 %switch, label %block2, label %block1 + +block1: ; preds = %entry, %block3 + %cond2 = icmp ugt i8 %x, 1 + br i1 %cond2, label %block3, label %block2 + +block2: ; preds = %entry, %block1 + br label %block3 + +block3: ; preds = %block2, %block1 + %cond3 = icmp eq i8 %x, 0 + br i1 %cond3, label %exit, label %block1 + +exit: ; preds = %block3 + ret void +} + +define void @pr165088_original(i8 %x) { +; CHECK-LABEL: define void @pr165088_original( +; CHECK-SAME: i8 [[X:%.*]]) { +; CHECK-NEXT: [[ENTRY:.*:]] +; CHECK-NEXT: [[TMP0:%.*]] = icmp ult i8 [[X]], 2 +; CHECK-NEXT: br i1 [[TMP0]], label %[[BLOCK2:.*]], label %[[BLOCK3:.*]] +; CHECK: [[BLOCK1:.*]]: +; CHECK-NEXT: [[COND3_OLD_OLD:%.*]] = icmp ugt i8 [[X]], 1 +; CHECK-NEXT: br i1 [[COND3_OLD_OLD]], label %[[BLOCK3]], label %[[BLOCK2]] +; CHECK: [[BLOCK2]]: +; CHECK-NEXT: br label %[[BLOCK3]] +; CHECK: [[BLOCK3]]: +; CHECK-NEXT: [[COND4:%.*]] = icmp eq i8 [[X]], 0 +; CHECK-NEXT: br i1 [[COND4]], label %[[EXIT:.*]], label %[[BLOCK1]] +; CHECK: [[EXIT]]: +; CHECK-NEXT: ret void +; +entry: + %cond = icmp ne i8 %x, 0 + %cond3 = icmp ne i8 %x, 0 + %or.cond = and i1 %cond, %cond3 + br i1 %or.cond, label %block3, label %block2 + +block1: ; preds = %block3 + %cond3.old = icmp ugt i8 %x, 1 + br i1 %cond3.old, label %block3, label %block2 + +block2: ; preds = %block1, %entry + br label %block3 + +block3: ; preds = %block2, %block1, %entry + %cond4 = icmp eq i8 %x, 0 + br i1 %cond4, label %exit, label %block1 + +exit: ; preds = %block3 + ret void +} |
andjo403 reviewed Oct 25, 2025
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In
simplifyBranchOnICmpChain, if we can merge the comparisons into a range check, use a conditional branch instead. This change also breaks the cycle found in #165088.Note: I cannot guarantee that such an infinite loop cannot be encountered after this patch. FWICT, we don't have a method to efficiently detect this bug for CFG optimizations (For peephole optimizations, see Termination-Checking for LLVM Peephole Optimizations).
Closes #165088.